When a zipper strip is applied to a film or web, heat and pressure are applied to effect the welding of the zipper to a film or web from which the bag or other container is to be made.
In order to secure the zipper to the film or web, the zipper is conventionally provided with a plurality of flanges on the two interengageable members which together make up the fastener. The most common practice is to provide four flanges, two on each member, in order to seal these flanges to film or web material on each side of the zipper. However, some zippers are provided with only three flanges, and some with only two. Sealing at the flanges is effected in order to keep heat and pressure away from the vulnerable male/female closure elements. However, it is a problem with such multi-flanged zippers that they tend to be bulky and do not easily deflect when used in cross-web procedures.